From 0b7526e977b6dd1209b2d043fbd7dc7a431852ce Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Guillermo=20Rodr=C3=ADguez?= <grodriguez@ingelabs.com>
Date: Wed, 4 Feb 2026 18:16:33 +0100
Subject: [PATCH] Work around clang hang compiling interp.c with -O1 and higher
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Clang's LLVM backend hangs indefinitely when compiling
src/interp/engine/interp.c with optimization level -O1 or higher.
The hang is caused by LLVM's TailDuplication pass encountering
exponential time complexity with the computed goto dispatch
pattern used in the interpreter. The problem can be reproduced
on Linux and macOS, and the actual cause was confirmed via
profiling.

Work around by adding -mllvm -tail-dup-limit=0 to the interp
CFLAGS when compiling with clang. This disables the problematic
pass (for interp.c only) while keeping all other LLVM
optimizations intact.

Signed-off-by: Guillermo Rodríguez <grodriguez@ingelabs.com>
---
 configure.ac | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/configure.ac b/configure.ac
index 3538043..9a02863 100644
--- a/configure.ac
+++ b/configure.ac
@@ -254,6 +254,26 @@ dnl Checks for programs.
 AC_PROG_CC
 AM_PROG_AS
 
+dnl Check if we're using clang - needed for workarounds
+AC_MSG_CHECKING([whether compiler is clang])
+AC_COMPILE_IFELSE(
+    [AC_LANG_PROGRAM([], [[
+        #ifndef __clang__
+            #error not clang
+        #endif
+    ]])],
+    [using_clang=yes],
+    [using_clang=no])
+AC_MSG_RESULT([$using_clang])
+
+dnl Clang's TailDuplication pass has exponential time complexity with the
+dnl computed goto dispatch pattern used in the interpreter.  Disable tail
+dnl duplication (for interp.c only) to prevent the hang while keeping all
+dnl other LLVM optimizations intact.
+if test "$using_clang" = yes; then
+    interp_cflags="$interp_cflags -mllvm -tail-dup-limit=0"
+fi
+
 AC_CHECK_PROGS(JAVAC, ecj jikes "gcj -C" javac)
 
 dnl Checks for libraries.